This research program aims to provide a better understanding of how basilar-membrane nonlinearities relate to speech recognition in noise and the extent to which a reduction or loss of nonlinearities for individuals with cochlear hearing los contributes to their communication difficulties. The unifying framework for the design of experiments and hypothesis testing is the active process in the cochlea, as revealed by measures of auditory function over a wide range of levels, using tonal and complex sounds, speech, and other sounds with fluctuating envelopes. In addition to high sensitivity and sharp tuning for low inputs, an important consequence of the active mechanism is the compressive nonlinearity in the basilar-membrane response. This process leads to level-dependent changes in masking, compression, tuning, and temporal effects, each of which has implications for speech recognition in noise. A thorough understanding of the negative consequences of reduced basilar-membrane nonlinearities is lacking due to the limited knowledge of nonlinear effects for complex sounds such as speech; small datasets from human subjects with large individual differences, especially for listeners with mild-to-moderate hearing loss; unknown effects of efferent activity; and uncertain interactions among level-dependent changes in masking, compression, and tuning. A comprehensive series of new experiments is planned within two broad aims, each with multiple components, to address key questions concerning the effects of basilar-membrane nonlinearities on auditory perception and recognition of speech. Aim 1 measures nonlinearity effects on compression and tuning to test the hypothesis that nonlinearities in the basilar-membrane response underlie level-dependent changes in speech recognition in noise. Growth of masking, temporal masking, and distortion-product otoacoustic emissions (Aim 1.1), auditory-filter bandwidths (Aim 1.2), and growth of masking for speech (Aim 1.3) will be measured over a wide range of input levels for large numbers of subjects with normal hearing and mild-to-moderate hearing loss. Aim 2 measures nonlinearity effects for sounds with fluctuating envelopes to test the hypothesis that level-dependent changes in the basilar-membrane response underlie changes in effective temporal envelopes with increases in level and play a role in detection and masking of fluctuating signals. Experiments in Aim 2 assess level-dependent changes in gap detection (Aim 2.1), modulation detection (Aim 2.2), and benefit from fluctuating maskers for speech recognition (Aim 2.3). To relate these results to basilar-membrane nonlinearities and disentangle interrelated nonlinear effects, level-dependent changes in compression and tuning will be measured and basilar-membrane responses inferred, as described in Aim 1, and supplemented by a well-established physiologically based computation model.